Iron-sulfur clusters and related species form the active sites of an important class of metalloenzymes that participate in electron transfer and catalysis in a wide variety of organisms. This project involves the use of novel techniques in quantum chemistry to study electronic structure problems in this active sites. The principal goal is to understand the nature of spin-coupling in these systems, and to explore how these couplings influence their oxidation-reduction and catalytic properties. This will be accomplished though systematic studies of various active site models in calculations of Heisenberg and resonance exchange parameters, Mossbauer and electron spin resonance parameters (including g and hyperfine tensors), electric fields and charge densities, and analysis of bond energies and their variation with cluster structure. All of the quantum calculations will use local density functional models expanded in large basis sets of atomic orbitals. Systems to be studied include: (1) 4Fe-4S and 4Fe-4Se clusters in the +1 and +3 oxidation states (a continuation of current efforts); (2) intermediates in the catalytic cycle of aconitase; (3) mixed-metal clusters with XFe3 stoichiometry, where X is Mo or Ni; (4) models for the active sites 3 of Rieske proteins. In addition, we plan to extend our current models to handle spin-orbit couplings (and hence zero-field splittings), to prepare new graphical representations of charge and spin densities, and to explore new models for electron transfer events in which ligand spin polarization is a mediating influence. As a whole, this study should provide valuable information about the spectroscopy and chemistry of iron sulfur and related clusters. We have established close collaborations with a variety of experimental groups to help ensure that the models we build are of real relevance to current problems in bioinorganic chemistry.